1. Field of the Invention
This invention relates to a device that measures a sample's optical properties, such as spectral reflectance or spectral transmittance, and particularly to a device which can perform high-precision measurement at all times regardless of the optical properties of the sample.
2. Description of the Prior Art
Conventionally, a device that measures the optical properties of a source and receiving the light from the measurement sample is generally and widely known. One example is a colorimetric device that measures the color of a sample by receiving the light reflected off or passing through the sample. The prior art and this invention will be explained below with reference to a colorimetric device, and particularly to a device that performs color measurement of a sample by receiving the light reflected off a sample. An explanation of various devices that receive light passing through a sample will be omitted because, other than a construction that leads the light from the sample to a measuring member, their construction is identical with that of the above device. In addition, this invention may be easily applied in the construction of a device that measures the sample's density and other optical properties because the device's construction is almost the same.
In a device that receives reflected light from a sample and measures the color, the sample is first irradiated with a white light source, etc. and the reflected light is dispersed into multiple basic wavelength components using a spectral filter, etc. Each of the dispersed wavelength components is then received by a photoelectric conversion element such as a photodiode, and the electric current or voltage proportionate to the luminous energy of the light is output as a measurement value. Incidentally, the output from this photoreceptor element is generally converted from analog to digital form so that it may be processed by a digital CPU, etc.
Now, because the luminous energy of the light received by the photoreceptor element is indicated by the product of the luminous energy of the light emitted by the light source and the reflectance of the measurement sample, the count value, which is the output of the photoreceptor element after A/D conversion, is proportionate to the luminous energy of the light emitted by the light source and the reflectance of the sample. In other words, the relationship given below exists: EQU (count value)=k.times.(luminous energy of the light emitted by the light source).times.(sample reflectance) (1),
where k is a proportional constant.
In addition, the resolving power per one count of count value is indicated by EQU (resolving power)=(reflectance)/(count value) (2)
In the conventional colorimetric device, when a sample having a relatively high reflectance, such as a white sample, is being measured, a photoreceptor element is selected so that the element output does not overflow, and an A/D conversion circuit constant is determined so that the circuit's counter does not overflow. For example, when designing a device such that the count value equals 10,000 when measuring a sample having 100% reflectance, the resolving power becomes EQU (resolving power)=100%/10.000=0.01% (3)
In other words, with this colorimetric device a reflectance up to a range of 0.01% can be measured.
Next, a case where measurement of the two types of sample shown in FIG. 6 is performed using this colorimetric device will be considered. Sample 1 is a sample with a maximum reflectance of 100%, while sample 2 is a sample having a reflectance which is one-tenth of that of sample 1 for each wavelength. Since the high-reflectance sample 1 having a maximum reflectance of 100% can be measured up to a range of 0.01%, the ratio between the maximum measurable reflectance and the minimum measurable reflectance is 10.000:1. However, since the low-reflectance sample having a maximum reflectance of 10% can be measured only up to a range of 0.01%, the ratio between the maximum measurable reflectance and the minimum measurable reflectance is 1000:1.
When measuring the color of a substance, since color is distinguished by device of whether the relative relationships between the various dispersed wavelength components are large or small, the larger the ratio between the maximum measurable reflectance and the minimum measurable reflectance is, the better the capability to distinguish color is. However, when measuring sample 2 as described above, the above ratio with respect to sample 1 is 1:10. In other words, the ability of the colorimetric device to distinguish color when measuring low-reflectance sample 2 is one-tenth of that present when measuring high-reflectance sample 1.
Consequently, in the conventional colorimetric device, the lower the sample's reflectance is, the smaller the ratio becomes between the maximum measurable reflectance and the minimum measurable reflectance, and the problem arises that the colorimetric device becomes accordingly less able to distinguish color.
With a low-reflectance sample as well, the resolving power may be increased in order to perform precise color measurement, and the A/D conversion circuit count value may be increased in order to increase the resolving power. As methods to increase the count value, the luminous energy of the light emitted by the light source may be increased, as is understood from equation (1) above, or the number of bits or the gain of the A/D conversion circuit may be increased in order to increase proportional constant k. However, in the conventional colorimetric device the light source constantly performs emission of the maximum luminous energy. Furthermore, in connection with proportional constant k, when the number of bits in the A/D conversion circuit is increased, the price of the circuit increases proportionally, resulting in higher manufacturing costs. In addition, when the gain is increased and measurement of a low-reflectance sample is performed, the S/N ratio of the output signal falls, and errors increase. Moreover, it requires that the gain be alternated when a high-reflectance sample is measured and when a low-reflectance sample is measured.
The present invention takes the above problems into account, and its object is to provide an optical properties measuring device which is capable of measurement with high accuracy at all times regardless of the sample's optical properties, such as high or low reflectance.